Motor fuel pumps in motor vehicles which serve to convey motor fuel, especially diesel fuel, between the tank and injection pump, are constructed as piston or membrane pumps in which a cylinder chamber is divided into a suction side and a pressure side by the piston or membrane having a tappet drive. In this construction, the tappet is driven in a straight line on a front end by an eccentric which lies outside of the pump. One problem with this arrangement resides in providing a reliable sealing of this straight line linkage. It is especially essential to prevent motor fuel from getting into the lubricating oil system. Also, the sealing must prevent the escape of motor fuel out of the pump since the fuel is inflammable at the hot motor block and in any event will cause environmental pollution problems. On the other hand, the sealing means are sensitive to heat so that a compromise between sealing effect, heat sensitivity and durability is necessary. The danger of a high heat load is very great due to the direct addition of the pump on the motor. Moreover, the hot motor oil is used for lubrication in the drive means of the pump.